1. Field of the Invention
The present invention relates to a bolus used closely contacting with a human body to correct distribution of absorbed dose in radiotherapy, and particularly to a disposable bolus.
2. Description of the Prior Art
Irradiation of rays such as X-rays, .gamma.-rays, and electron rays to a human body has been widely used for therapy against disease such as cancer. When the rays are irradiated on material, generally, dose of the rays decreases with depth, but the dose of the scattering rays whose directions are various increases. Therefore, the dose distribution of rays decreases exponentially with depth as shown in FIG. 6 and FIG. 7. But in rays with high energy the recoiling electrons and/or scattering rays have the components of forward direction mainly, and side scattering rays decrease. Therefore, the dose of rays becomes the greatest at a certain depth compared with surface dose, and after that the dose of rays decreases exponentially with depth as shown in FIG. 6 and FIG. 7. In therapy to treat the effect of these rays on human skin, the therapy sometimes causes adverse effects because of irradiation of unnecessary rays to normal tissues other than lesions.
For example, assuming that there is a focus under the skin surface in the position of 5 mm as shown in FIG. 1. If rays whose absorbed dose serves as the maximum under the skin surface in the position of 10 mm are irradiated to this focus to cure the focus, the unnecessary and excess rays are irradiated to normal tissues other than the focus. Especially, in the position of 10 mm under the skin surface the absorbed dose is the greatest, therefore it is risky to cause this part to accompany radiation damage. The bolus has a function to control the dose distribution of rays to irradiate the greatest absorbed dose to the focus. The bolus is used by making it intervene between an irradiation apparatus and a human body as shown in FIG. 2. Using a bolus as mentioned above, it is possible to cure lesions by irradiating a required therapeutic dose of rays to lesions efficiently.
If radiotherapy is performed to a tumor in the brain as shown in FIG. 3A, the dose distribution of rays in the brain is a spherical distribution reflecting an outward form of a head. In this situation, the dose distribution of the greatest absorbed dose is also a spherical distribution to the tumor which have a plate-like form as shown in FIG. 3A. Therefore, it is very dangerous that the greatest absorbed dose is irradiated to a part except the tumor as shown in FIG. 1. Here using the bolus which has a form fitted to an outward form of a head as shown in FIG. 3B, it is possible to correct the spherical distribution of rays.
Generally, a practically usable bolus must satisfy at least the following properties and conditions:
1. It is a substance equivalent to human body tissue. PA1 2. It is homogeneous. PA1 3. It has excellent plasticity, appropriate resilience, and excellent form-compatibility and adhesiveness to a living body. PA1 4. It is non-toxic. PA1 5. There are no changes in energy, etc. PA1 6. It has even thickness. PA1 7. It does not contain air.
Moreover, it is desirable to have the transparency in addition to the above properties and conditions. Although the expression "equivalent to human body tissue" means in a strict manner that the atomic composition is the same as that of a human body, it means in this context that properties in terms of absorption and scattering of rays are the same as those of substantial tissue.
Plastics, paraffin, synthetic rubbers, silicone, water, and the like have been used as bolus materials up to date.